The present invention relates generally to polymer foam processing and, more particularly, to a system and method for delivering blowing agent in a polymeric foam process.
Polymeric materials are processed using a variety of techniques. Many techniques employ an extruder which includes a polymer processing screw that rotates within a barrel to plasticate polymeric material. Some processing techniques, such as injection molding and blow molding, may be discontinuous. That is, during operation, the screw does not plasticate polymeric material continuously. Discontinuous processes may be repetitive cycles which include a plastication period, in which the screw rotates and polymeric material is accumulated, followed by an injection (or ejection) period, in which the screw does not rotate and the accumulated polymeric material is injected into a mold (or ejected through a die).
Polymeric foam materials, including microcellular materials, may be processed by injecting a physical blowing agent into the polymeric material within the extruder through a blowing agent port in the barrel. Many conventional blowing agent delivery systems inject blowing agents continuously into the polymeric material within the barrel. In discontinuous processes, including certain injection molding and blow molding processes, such continuous delivery systems may cause a lack of control over the percentage of blowing agent injected into the polymeric material and may lead to an uneven distribution of the blowing agent in the polymeric material. In particular, the polymeric material in the vicinity of the blowing agent port, when the screw ceases to plasticate polymeric material, may contain higher amounts of blowing agent because of its increased residence time in proximity with the blowing agent injection port. The uneven distribution of blowing agent may result in viscosity variations within the polymeric material in the extruder which can cause output inconsistencies and other problems. Such effects may generally reduce control over the process and may narrow the processing window.
In some polymer processes, including some discontinuous processes, such conventional blowing agent delivery systems may be adequate. However, in other processes, such as discontinuous processes that require relatively precise control over blowing agent delivery, the conventional delivery systems may impair the process for one or more of the reasons described above. In particular, certain processes for producing microcellular foam may be adversely affected if the blowing agent is not precisely controlled.
Accordingly, a need exists for a system that delivers blowing agent in a controlled manner to a discontinuous polymer processing system.
The invention provides a blowing agent delivery system and method for introducing a blowing agent into a polymeric foam processing system. The delivery system is designed to introduce blowing agent into polymeric material within an extruder when polymeric material is being plasticated, for example, during the plastication period of an injection molding cycle. The rate of introduction of blowing agent into polymeric material may be coupled to the flow rate of polymeric material in the extruder to ensure precise control over blowing agent percentage in the resulting mixture of polymeric material and blowing agent. The delivery system is also designed to prevent the introduction of blowing agent into the polymeric material when polymeric material is not being plasticated, for example, during the injection period of an injection molding cycle. The delivery system may be used in conjunction with discontinuous systems including injection molding or blow molding systems.
One aspect of the invention provides a polymeric foam processing system operable to, in a repetitive cycle, plasticate polymeric material and eject an accumulated mixture of polymeric material and blowing agent through an outlet. The system includes an extruder having a polymer processing screw constructed and arranged to rotate within a barrel, during a plastication period of the cycle, to convey polymeric material in a downstream direction at a flow rate within a polymer processing space defined between the screw and the barrel. The extruder includes a blowing agent port formed in the barrel. The system further includes a blowing agent delivery system having an outlet fluidly connected to the blowing agent port. The blowing agent delivery system is constructed and arranged to repeatedly introduce a fixed mass of blowing agent into the polymeric material in the polymer processing space, during the plastication period of the cycle, at a rate coupled to the flow rate of polymeric material, to form a mixture of polymeric material and blowing agent therein.
Another aspect of the invention provides a polymeric foam processing system. The system includes an extruder including a barrel having a blowing agent port. The barrel is constructed and arranged to mount therein a screw rotatable, in a first mode of operation, to convey polymeric material at a flow rate in a downstream direction within a polymer processing space defined between the screw and the barrel. The system further includes a chamber fluidly connectable to the blowing agent port. The chamber is constructed and arranged to confine a fixed amount of blowing agent in an enclosed volume. The system further includes a piston housed within the chamber and moveable, at a rate coupled to the flow rate of polymeric material, in a direction decreasing the enclosed volume to inject the fixed amount of blowing agent through the blowing agent port to form a mixture of polymeric material and blowing agent in the polymer processing space.
Another aspect of the invention provides a polymeric foam processing system operable to, in a repetitive cycle, plasticate polymeric material and eject an accumulated mixture of polymeric material and blowing agent through an outlet. The system includes an extruder having a polymer processing screw constructed and arranged to rotate within a barrel, during a plastication period of the cycle, to convey polymeric material in a downstream direction at a flow rate within a polymer processing space defined between the screw and the barrel. The extruder includes a blowing agent port formed in the barrel. The system further includes a blowing agent delivery system having an outlet fluidly connected to the blowing agent port. The blowing agent delivery system including a chamber designed to confine a fixed mass of blowing agent in an enclosed volume and a piston mounted within the chamber. The piston defining, in part, the enclosed volume of the chamber, and moveable from a first position to a second position to force blowing agent through the outlet of the chamber and the blowing agent port and into the polymeric material, during the plastication period, to form a mixture of polymeric material and blowing agent in the polymer processing space. The blowing agent delivery system being equipped to measure temperature and pressure of the blowing agent confined in the chamber. The system further including a controller in communication with the blowing agent delivery system. The controller designed to receive the inputs of the temperature and pressure of the blowing agent confined in the chamber from the blowing agent delivery system, and an input of a pre-determined mass of blowing agent. The controller being capable of calculating a pre-determined volume of blowing agent from the inputs. The controller designed to send a signal to the blowing agent delivery system to move the piston to the first position to set the enclosed volume of the chamber equal to the pre-determined volume and to move the piston to the second position at a rate coupled to the flow rate of polymeric material.
Another aspect of the invention provides a system. The system includes a blowing agent chamber, fluidly connectable to a blowing agent port in a polymer processing apparatus. The polymer processing apparatus is constructed and arranged to include a screw mounted in a barrel. The screw is rotatable as polymeric material flows at a flow rate in a downstream direction within a polymer processing space defined between the screw and the barrel. The chamber has a variable volume and is constructed and arranged to confine a fixed amount of blowing agent therein. The system is constructed and arranged to reduce the chamber volume at a rate coupled to the flow rate of polymeric material.
Another aspect of the invention provides a system. The system includes a blowing agent chamber, fluidly connectable to a blowing agent port in a polymer processing apparatus. The polymer processing apparatus is constructed and arranged to include a screw mounted in a barrel. The screw is rotatable as polymeric material flows at a flow rate in a downstream direction within a polymer processing space defined between the screw and the barrel. The system further includes a controller connectable to the blowing agent chamber, constructed and arranged to reduce the chamber volume at a rate coupled to the flow rate of polymeric material.
Another aspect of the invention provides a method of introducing blowing agent into a polymer processing system. The method includes arranging a polymer processing system to, in a repetitive cycle, plasticate polymeric material within an extruder and eject an accumulated mixture of polymeric material and blowing agent through an outlet. The method further includes accumulating a pre-determined mass of blowing agent in a chamber of a blowing agent delivery system. The method further includes introducing the pre-determined mass of blowing agent from the chamber through a blowing agent port in the extruder into the polymeric material, during a plastication period of the cycle, at a rate coupled to the flow rate of the polymeric material in the extruder.
Among other advantages, the blowing agent injection system can introduce blowing agent discontinuously into a discontinuous polymer processing system. The rate of blowing agent introduction may be coupled to the flow rate of polymeric material in the extruder when polymeric material is being plasticated. When polymeric material is not being plasticated, the delivery system does not introduce blowing agent into the polymeric material in the extruder. As a result, an even distribution of a selected amount of blowing agent in the polymeric melt, for example based on the weight of polymeric material, may be achieved. The delivery system, thus, significantly reduces or eliminates viscosity variations of the polymeric material and blowing agent mixture which may arise in conventional blowing agent delivery systems that continuously introduce blowing agent into a discontinuous processing system.
The blowing agent injection systems according to the invention are particularly useful in processes that benefit from the precise control of the metering of blowing agent into the polymeric melt, such as certain processes for producing microcellular materials. In particular, microcellular materials having an average cell size of less than 100 microns may be produced using the blowing agent delivery system in conjunction with injection molding or blow molding processes. Furthermore, the blowing agent delivery system has a simple design and may be produced relatively inexpensively.